Current commercially available three-dimensional (3D) time of flight (TOF) cameras use in general an array of light emitting diodes (LEDs) with micro-lenses that more or less have a similar field of illumination (FOI) as the field of view (FOV) covered by the camera. Laser-based 3D TOF cameras also require a specific micro-optical design to generate a FOI that matches to the FOV of the camera.
LEDs are commercially available as surface-mount devices. They can be ordered either without any micro-lens or with micro-lens. A micro-lens is required for narrower emission angles. With respect of 3D sensing, such devices have two main drawbacks:                In general, the emission field has circular shape. On the other side, the imaged area on the image sensor, which determines together with the lens the FOV, corresponds to a rectangular-shaped area, or a square-like area distorted by the lens to form a barrel, pincushion distortion.        The commercial availability of different emission angles is limited. In order to get a good match of FOI and FOV, expensive custom-made light source micro-optics have to be developed.        
A typical FOI pattern is shown in the simulation in FIGS. 1 and 2. They show the intensity distribution on a 2×2 m (meter) target at a distance of 1 m from the LED light source. The two pairs of lines indicate the FOV of this specific camera in the horizontal and vertical directions in FIG. 1. In FIG. 2, the rectangle shows the FOV. Especially in the corners, FOV is not well illuminated while illumination power is wasted above and below the FOV.
Narrow FOI has the advantage that most of the emitted light gets onto the FOV. However, the uniformity over the area gets lost. Usually, the light power in the center of the FOV is larger than the power at the corners. On the other side, a wide FOI shows an improved uniformity over the entire FOV, but a lot of power is wasted outside the FOV.
Other problems can arise when the FOI is wider than a FOV. It might happen that objects outside the FOV reflect modulated light onto objects in the FOV. In such a case, the receiver gets a resulting image from the direct path and the indirect path. Not only does this potentially disturb the actual range measurement of the object in the FOV, it also cannot be detected because the reflecting object is outside the FOV and cannot be seen by the camera.